Gripping device

ABSTRACT

A device for gripping test specimens or workpieces. The device includes a body having an aperture therein for receiving one end of the specimen or workpiece. Means are mounted around the one end for translating axial forces into radial inward gripping forces. Means are mounted around the specimen or workpiece, adjacent the translating means and removably secured to the body for applying the axial forces to said translating means so that the inward radial forces are transmitted from the translating means to the specimen or workpiece for positively gripping the specimen or workpiece about its periphery.

, H 0 u 1 fl h l J i United States Wat-tent [is] anson Anderson [45]their, 22, 1972 [54] GWHPIPHNG DIEVlI C E 3,427,036 2/1969 Siebelhoff..279/4 [72] inventor: Philip .ll. Anderson, Deerfield, llll. 432 [73]Assignee: institute oi Gas Technology 3,016,250 1/1962 Franck .285/342[22] Filed: 1969 Primary Examiner-Richard C. Queisser [2]] APPL myg -mAssistant Examiner-Jlohn Whalen Altt0rneyBa1r, Freeman 6: Molmare [52]lU..=Cl 733/103, 279/47 57 mgmmr 'r [51] llnt. Cll. 60hr 3/00 [58] manflilfiemmlil ..73/103;279/46,47,48,4; A i F gYIPPIg F speclmens f T285/245 device includes a body having an aperture therein for receivingone end of the specimen or workpiece. Means are [56] mlgfiwmms monntedaround tl e one end for translating axial forces into radial inwardgripping forces. Means are mounted around the UNITED STATES PATENTSspecimen or workpiece, adjacent the translating means and removablysecured to the body for applying the axial forces to 2,755,094 7/1956Ben amm "279/51 i translating means so that the inward radial forces are2,956,826 10/1960 transmitted from the translating means to the specimenor 2,982,557 5/ 1961 Anscliutz.... ..279/57 workpiece f i i l grippingthe specimen or workpiece 3,246,903 4/1966 Sattler .279/47 aboutitsperiphery 3,335,603 8/1967 Gram .73/103 3,365,204 1/ 1968 Benjamin eta1. ..279/47 3 Claims, i Drawing Figures PATENTEUFEB22 m2 WIN 1 [IF 2ENTOR PHILIP J. ANDEQSON INV ELM-35Gb GRIUPPING DEVICE BACKGROUND OF THEINVENTION Field of the Invention and Description of the Prior Art Thisinvention relates to a device for gripping various specimens andworkpieces and it particularly relates to a device for gripping coredand cast specimens for tensile testing purposes.

There is presently no satisfactory device or apparatus for holding orgripping the ends of cylindrical samples of concrete, rock, mortar,etc., for subjecting them to tensile tests. In fact, there is nostandard tension test for concrete, rock, or mortar which is presentlyaccepted by The American Society for Testing Materials since all theknown tensile procedures have certain drawbacks.

In one tensile testing procedure, cast briquette specimens are used andspecial grips are required for holding them during testing. Onedisadvantage of this method is that no testing of cored rock or concreteis possible because a specific shape is required for the briquettes. Afurther significant drawback of this test procedure is that tests haveshown that it is not possible to guarantee a true axial pull. Thus, thebreaks which occur do not represent pure tensile stress but also involveshearing stress so the tensile strength cannot be accuratelyascertained.

In another tensile test procedure, cylindrical specimens are used andthreaded bolts are embedded in the ends of the specimens. The threadedbolts are cast into the specimen and the exposed ends are gripped by theself-centering chucks ofa tensile testing machine. A disadvantage ofthis procedure is that failures commonly occur by splitting, rather thanby tension, as a result of high stress concentration at the interfacebetween the bolt threads and the specimen. Also, this procedure can beused only with cast specimens and cannot be used with cored specimens.

Still another tensile test procedure involves the use of steel endplates which are bonded to cylindrical test specimens by use of an epoxyresin or other suitable adhesive. The end plates are drilled and tappedand threaded bolts are screwed into the tapped holes. The exposed endsof the bolts are then gripped by the chucks of the tensile testingmachine. This technique has problems in sample preparation. Althoughthis technique is useful with both case and cored specimens, the ends ofthe specimens must be flat and parallel with each other. Then, the endplates must be centered on the specimen so that the threaded bolts arecoaxial with the specimen. These requirements result from the fact that,if the bolts are not coaxial with the specimen, the applied stress isnot a pure tensile stress and the values obtained in testing do notrepresent the true and accurate tensile strength of the test piece.Preparation of the ends of the samples to obtain the required coaxialalignment of the end plates with the specimen, construction of jigs toobtain alignment, and the time required for setting the epoxy cement aresome of the disadvantages of this test procedure.

SUMMARY OF THE INVENTION It is therefore an important object of thisinvention to provide an improved device for gripping test specimens orwork pieces and particularly for substantially avoiding thedisadvantages of prior devices for gripping specimens for tensiletesting purposes.

It is a further object of this invention to provide an improved devicefor gripping objects for tensile testing wherein a true axial pull ofthe test specimen is accomplished.

It is still a further object of this invention to provide an improveddevice for gripping a test specimen or workpiece wherein rapid and easyattachment is provided between the specimen and the gripping device.

It is another object of this invention to provide an improved tensiletesting device which is useful with both cast and cored samples.

It is yet another object of this invention to provide an improvedgripping device wherein the grips may easily be constructed toaccommodate samples or workpieces having various sizes and shapes.

It is still another object of this invention to provide improved devicesfor gripping test specimens or workpieces wherein the grips arecharacterized by their simplicity and economy of construction and use.

Further purposes and objects of this invention will appear as thespecification proceeds.

The foregoing objects are accomplished by providing a device forgripping test specimens or workpieces wherein the device includes a bodyhaving an aperture therein for receiving one end of the specimen orworkpiece, means carried by one end of the specimen or workpiece fortranslating axial forces into radial inward gripping forces which actagainst the specimen or workpiece, and means are mounted around the testspecimen or workpiece and being adjacent said translating means andremovably secured to said body, said last means being for applying axialforces to said translating means whereby the inward radial forces aretransmitted from the translating means to the specimen or workpiece forpositively gripping the specimen or workpiece about its periphery.

BRIEF DESCRIPTION OF THE DRAWINGS Particular embodiments of the presentinvention are illus trated in the accompanying drawings wherein:

FIG. l is a side elevational view of a test specimen having the improvedgripping devices positioned on its opposite ends for tensile testpurposes;

FIG. 2 is a fragmentary, enlarged partially sectioned view of the end ofa test specimen locked into positive engagement with one of the grippingdevices made in accordance with the invention;

FIG. 3 is an exploded view of the components of the improved grip forholding test specimens or workpieces wherein the loading gland ismechanically and manually operated; and

FIG. 4 is an exploded view of an alternate embodiment of a grippingdevice of my invention, wherein the loading gland is hydraulicallyoperated.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the description of theinvention, reference will be made to my improved gripping device fortensile testing of various specimens, particularly concrete, mortar, orrock specimens. It is to be understood, however, that the device may beused for a variety of purposes including use in gripping a workpiece invarious operations such as machining, drawing, elongating, twisting, andthe like. My improved gripping device finds a particularly usefulfunction as a gripping device for tensile testing purposes. For thisreason, the detailed description of the invention is specificallydirected to tensile testing.

Referring to FIGS. 11-3, my improved gripping device, generally It), forreceiving the opposite ends of a test specimen 12 has four main parts.These four parts include a body 114, afrustoconical deformable ferrule16, a rigid ring I8, and a rigid loading gland 20.

The body I4 is provided at one end with a cylindrical well 22 forreceiving one end portion 2d of the test specimen l2. Although the testspecimen is cylindrical in shape and circular in cross section, it is tobe understood that the particular shape and cross section of thespecimen may be varied, provided that the parts of the device It) inintimate contact with the test specimen have the same cross-sectionalshape to provide the desired intimate contact. The outer end portion ofthe well 22 defines a frustoconical recess 26. The opposite end of thebody Id has a reduced shank 28 for receiving the self-centering chuck ofa standard tensile test machine (not shown). The central portion 30 ofthe body M is desirably hexagonal or the like in external cross sectionfor receiving a wrench or the like so that the loading gland 2t may bemanually tightened on the threaded outer end 32 of the elongated body M.

A deformable plastic ferrule 16 having a frustoconical outer peripheryis received on the periphery of the outer end portion 24 of the testspecimen 12 and is received within the frustoconical recess 26 at theouter end of the well 22 in surface to surface relationship. The ferrule16 surrounds the end 24 of the specimen 12 as does the adjacent rigidbackup ferrule 18. The inner surfaces of both the deformable ferrule 16and the rigid ferrule 18 are cylindrical in shape so as to be snuglyreceived by the outer periphery of the cylindrical specimen 12. Theouter end (or end opposite the body 14) of the plastic ferrule 16 isprovided with a chamfered or tapered recess 38 which receives anadjacent chamfered or tapered outer periphery on the rigid ferrule 18.It is important in the construction of our device that the ferrule 16 beconstructed of suitable deformable plastic material.

As seen in FIG. 2, when the device 10 and specimen 12 are ready fortesting, the frustoconical deformable ferrule 16 is received within thefrustoconical recess 26 of the well 22 of the body 14. Also, the rigidferrule 18 is positioned adjacent the outer side of the deformableplastic ferrule 16.

The loading gland 20 is received on the threaded outer end 32 of thebody 14. A central opening 40 in the outer end ofthe loading glandpermits the passage of the cylindrical test specimen 12 therethrough. Anannular inwardly extending ridge or annular lip 42 bears against therigid ring or ferrule 18 with an axial force when the loading gland 20is secured on the body 14.

Although the operation of the device 10 shown in FIGS. 1-3 is believedto be clear from the foregoing, a brief description of the manner ofusing the device 10 will be provided. The plastic ferrule 16 is slidover the end 24 of the test specimen 12 so that the narrow end thereofpoints inwardly to be received in the recess 26 of the body 14. Therigid ferrule 18 is positioned around the end 26 of the specimen 12 andon the side of the ferrule 16 that is opposite the narrow end thereof.The loading gland 20 is inserted around the specimen 12 so that itsthreaded inner periphery is received by the threaded end 32 of the body14. The annular lip 42 bears against the annular outer end of the rigidferrule 18. When the ferrules 16 and 18 and the loading gland 20 are inproper position on the specimen 12, the end 24 of the specimen 12 isinserted into the well 22. The loading gland 20 is then threaded ontothe body 14. As the gland 20 is tightened, the annular lip 42 presses oraxially bears against the annular end of the rigid ferrule 18 with anaxial force and the rigid ferrule 18 in turn provides an axial forceacting on the deformable plastic ferrule 16, so as to force it into theannular recess 26 of the well 22.

Due to the combined effect of the conical exterior periphery of theplastic ferrule 16, the interior well 22 of the body 14, and thecylindrical shape of the inner surface of the plastic ferrule 16, theapplied load from the loading gland 20 upon the body 14 centers the testspecimen 12 so that the axes of the specimen 12 and the body 14 aresubstantially coaxial. The loading of the loading gland 20 applies anaxial force on the annular end of the rigid ferrule 18 or 43 which inturn transmits an axial force to the deformable plastic ferrule 16. As aresult of the frustoconical outer shape of the deformable ferrule 16,the axial force is translated into an inward radial force which actsequally against the specimen 12 and against the inner surface of thewell 22. This ultimately causes deformation of the plastic ferrule 16 soas to provide a uniform peripheral gripping load on the sample. When twoof the gripping devices 10 are placed on opposite ends of a specimen 12,as shown in FIG. 1, and when the shanks 28 of the gripping devices 10are placed in the test machine, tensile stress is applied to thespecimen 14. While the tensile force is being applied to the specimen12, the axial deformation of the plastic ferrule 16 is restrained by therigid backup ferrule 18 so that the radial pressure exerted against thespecimen by the plastic ferrule 16 increases.

Referring to the alternate embodiment shown in FIG. 4, the grippingdevice 100 again includes four basic parts-a body 102, a deformableferrule 104, a rigid ferrule 106, and a loading gland 108. The grippingdevice differs from the gripping device 10 in the provision of hydraulicpressure for applying axial force to the deformable ferrule 104. The useof hydraulic pressure provides certain advantages. First, greater axialforces can generally be provided by hydraulic means than by mechanicalforce applied to a loading gland from a wrench. Secondly, and of evengreater importance, the use of hydraulic pressure enables a constantgripping force to be provided, regardless of deformation of the plasticferrule 104 or of the specimen itself.

The body 102 is of substantially the same construction as the body 12 inthe embodiment shown in FIGS. 1-3. The body 102 includes a central wellfor receiving the end of a test specimen. The well 110 has afrustoconical outer end portion 112, the outermost portion having theenlarge diameter. A threaded outer periphery 114 is provided on the endof the body 102. The plastic deformable ferrule 104 has a frustoconicalouter periphery 116 which is designed to move into surface to surfacecontact with the frustoconical end portion 112 of the body 102. Thedeformable ferrule 104 has a central aperture 118 aligned with the well110 and the body 102. The aperture 118 has a frustoconical outer endportion 120.

The rigid ferrule 104 is different from the rigid ferrule in theembodiment of FIGS. 1-3. The rigid ferrule 104 is construced as amovable piston and cooperates with the loading gland 108 which has anannular cylindrical pressure chamber 122 formed therein. The rigidferrule 106 has a cylindrical outer periphery 124 which is receivedwithin the annular pressure chamber 122. The outer periphery 124includes an O-ring hydraulic seal which slides and seals against theouter wall of the chamber 122. The inner cylindrical periphery 128 ofthe right ferrule 106 slides against the inner wall 130 defining thepressure chamber 122 and includes an O-ring seal 129. The lower or innerend portion 132 includes a frustoconical outer periphery 134 which movesinto surface to surface mating contact with the frustoconical outerperiphery 124 on the plastic ferrule 104. The central aperture 136 iscoextensive and coaxial with the well 110, the aperture 118 and thedeformable ferrule 104.

The loading gland 108 includes a threaded inner portion which threadablyengages the threaded end 114 of the body 102. The central aperture 138again is coextensive with the well 110, with the aperture 118 of theplastic ferrule 104, and with the aperture 136 of the rigid ferrule 106.The test specimen (not shown) passes through these apertures and isinserted into the well 1 10.

An hydraulic inlet line 140 is provided in the loading gland 108. Thisinlet 140 is connected by suitable pressure seal means (not shown) witha source of hydraulic fluid pressure (not shown).

In operation of the embodiment of FIG. 4, when the test specimen isreceived by the well 110 and the rigid ferrule 106 bears against theplastic deformable ferrule 104 by securing of the loading gland 108 tothe body 102, hydraulic fluid is passed into the inlet 140. Thehydraulic fluid acting in the pressure chamber 102 drives thepiston-rigid ferrule 104 downwardly or applies axial pressure thereto.This axial pressure causes the rigid ferrule 106 to move against theplastic deformable ferrule 104. Because of the frustoconical outerperiphery of the deformable ferrule, the axial force imparted by thepiston-ferrule 106 is translated into an inward radial force which bearsagainst the outer periphery of the test specimen.

In principle, it is seen that the operation of the embodiment of FIG. 4is substantially the same as the operation of the embodiment of FIGS.1-3. The difference lies in the use of hydraulic pressure for applyingthe necessary axial force, which force is ultimately translated into aradial gripping force. The hydraulic pressure can be controlled toapplying constant pressure in the chamber 122 so that regardless ofdeformation of the workpiece or of the plastic ferrule, a substantiallyconstant axial force and thereby a substantially constant radial forceacts against the workpiece.

From the foregoing, it is seen that ali of the objections previously setforth have been accomplished. The device provides substantial advantagesover prior art gripping devices. For tensile test purposes, both coredand cast specimens may be tested for tensile strength by the use of thedevice it) or the alternate embodiment Mill. The gripping device, asdescribed, assures that only the axial force on a specimen is tested.The gripping devices are readily attached and used without any specialpreparation. Both the gripping device it] and the gripping device 100may be constructed to accommodate any desired size, shape, or crosssection of a test specimen or workpiece. The gripping device describedis useful generally for the secure gripping of an article which is to betested or is to be machined or is to have various operations performedon it,

While in the foregoing there has been provided a detailed description ofparticular embodiments of the present invention, it is to be understoodthat all equivalents obvious to those having skill in the art are to beincluded within the scope of the invention as claimed.

What I claim and desire to secure by Letters Patent is:

i. A method for tensile testing a brittle test specimen, such asconcrete, mortar and rock, said specimen having a central axis,comprising the steps of providing a pair of deformable Al'n nonrigidferrules, the entire peripheral internal shape of said ferrules beingthe same as the entire outer peripheral shape of said specimen, mountingone of said ferrules around each of the opposite ends of said specimento completely encompass each of said ends with one of said ferrules,confining each of said ferrules in a space formed by said specimen andfirst and second relatively axially movable members and having a volumesubstantially equal to the volume of each of said ferrules, moving saidfirst and second members axially to thereby apply an axial force to eachone of said confined deformable ferrules, translating said axial forceinto an inwardly directed radial force with said deformable plasticferrules acting against the said outer periphery of each end of saidspecimen for providing a radial inward gripping load of at least apreselected level on each of said ends, and applying a true tensileforce to said first and second members which is transmitted through saidferrules to said test specimen along its central axis.

2. The method of claim ll wherein said radial inward gripping loadincreases as said tensile force increases.

3. The method of claim 1 wherein said radial gripping load issubstantially constant.

1. A method for tensile testing a brittle test specimen, such asconcrete, mortar and rock, said specimen having a central axis,comprising the steps of providing a pair of deformable nonrigidferrules, the entire peripheral internal shape of said ferrules beingthe same as the entire outer peripheral shape of said specimen, mountingone of said ferrules around each of the opposite ends of said specimento completely encompass each of said ends with one of said ferrules,confining each of said ferrules in a space formed by said specimen andfirst and second relatively axially movable members and having a volumesubstantially equal to the volume of each of said ferrules, moving saidfirst and second members axially to thereby apply an axial force to eachone of said confined deformable ferrules, translating said axial forceinto an inwardly directed radial force with said deformable plasticferrules acting against the said outer periphery of each end of saidspecimen for providing a radial inward gripping load of at least apreselected level on each of said ends, and applying a true tensileforce to said first and second members which is transmitted through saidferrules to said test specimen along its central axis.
 2. The method ofclaim 1 wherein said radial inward gripping load increases as saidtensile force increases.
 3. The method of claim 1 wherein said radialgripping load is substantially constant.